The present invention relates to an exposure apparatus and method used for transferring a pattern on a mask onto a photosensitive substrate through a projection optical system in photolithography processes to produce, for example, semiconductor devices, liquid-crystal display devices, imaging devices (e.g. CCDs), or thin-film magnetic heads. More particularly, the present invention relates to a step-and-scan or other type of scanning exposure apparatus and scanning exposure method, in which exposure is carried out by synchronously scanning a mask and a photosensitive substrate relative to a projection optical system.
In photolithography processes for producing semiconductor devices or the like, stepper-type projection exposure apparatuses have mainly been used, in which a pattern on a reticle (or a photomask or the like) is transferred onto each shot area on a wafer (or a glass plate) by one-shot exposure through a projection optical system. Recently, however, attention has been paid to scanning exposure type projection exposure apparatuses, i.e. slit scan type and step-and-scan type projection exposure apparatuses, in which a reticle and a wafer are scanned relative to a projection optical system, thereby sequentially transferring a pattern on the reticle onto each shot area on the wafer, in order to comply with the demand that a pattern of a wider area should be exposed with the image-formation characteristics maintained at the desired level. It is possible according to the scanning exposure type of projection exposure apparatus to use approximately the largest diameter of the effective exposure field of the projection optical system by illuminating the reticle with slit-shaped illuminating light, by way of example. Moreover, by synchronously scanning the reticle and the wafer, the exposure field can be enlarged without being restricted by the optical system. Furthermore, because only a part of the effective exposure field of the projection optical system is used, the required accuracies for illuminance uniformity, distortion, etc. can be readily obtained.
Overlay accuracy is one of important performances of the above-described projection exposure apparatuses. More specifically, a semiconductor device, for example, is formed by stacking on a wafer a multiplicity of layers of different circuit patterns. Therefore, a circuit pattern drawn on a reticle must be precisely overlaid on a pattern already formed on each shot area on the wafer. One of the important factors influencing the overlay accuracy is the magnification error (distortion) of the projection optical system. There is a growing tendency for the size of patterns used in very-large-scale integrated circuits (VLSI) and so forth to shrink year by year. Accordingly, the demand for an improvement in the overlay accuracy on a wafer is also growing. Therefore, there is an extremely strong need to maintain the projection magnification at a predetermined value.
Incidentally, the projection magnification of the projection optical system fluctuates in the vicinity of a predetermined magnification in accordance with a slight temperature change in the system, a slight pressure variation or temperature change of atmospheric air in a clean room where the projection exposure apparatus is placed, and the irradiation history of the projection optical system irradiated with exposure light energy. Distortion may, for example, be produced by thermal expansion of the reticle. Therefore, some of the latest scanning exposure apparatuses have a magnification correction mechanism for finely adjusting the magnification of the projection optical system to realize a predetermined magnification. As specific examples of the magnification correction mechanism, the following mechanisms have already been proposed: a mechanism that changes the spacing between the reticle and the projection optical system; a mechanism that changes a predetermined lens spacing in the projection optical system; and a mechanism that adjusts the pressure in a predetermined gas chamber provided in the projection optical system.
Thus, various magnification correction mechanisms have heretofore been proposed to cope with a temperature change in the projection optical system of a projection exposure apparatus and other variations in environmental conditions. However, the conventional correction mechanisms are adapted to correct the distortion or magnification over the whole exposure area by calculating an amount of thermal expansion of the reticle or an amount of change in the distortion of the projection optical system. This is the same in scanning exposure type projection exposure apparatuses. That is, the coefficient of thermal expansion of the reticle has heretofore been regarded as uniform over the whole reticle, and the conventional practice is to correct the magnification of a projected image in a slit-shaped exposure area.
In the case of the scanning exposure type, however, the coefficient of thermal expansion of the reticle is not uniform over the whole pattern area in practice but varies according to the position in the scanning direction. Therefore, as the position of the reticle changes during scanning exposure, the distortion or magnification of the projected image in the slit-shaped exposure area on the wafer changes, and as a result, the overlay accuracy is degraded.
In view of the above-described circumstances, an object of the present invention is to provide a scanning exposure apparatus and scanning exposure method which provide superior image-formation characteristics irrespective of the position of the reticle in the scanning direction.
The present invention provides a scanning exposure method including the step of synchronously scanning a mask and a substrate relative to illuminating light, thereby transferring an image of a pattern on the mask onto the substrate through a projection optical system by scanning exposure, and the step of changing, during the scanning exposure, an image-formation characteristic of the projection optical system according to a scanning position of the mask.
The image-formation characteristic of the projection optical system may include at least one of magnification and distortion.
The change of the image-formation characteristic may be effected by moving a lens element of the projection optical system for projecting the image of the pattern on the mask onto the substrate.
The change of the image-formation characteristic may be effected by adjusting a refractive index in an enclosed space provided in the projection optical system for projecting the image of the pattern on the mask onto the substrate.
The image-formation characteristic of the projection optical system may be changed by taking into consideration thermal expansion of the mask, the thermal expansion varying in amount according to the position on the mask.
The scanning exposure method may further include the step of obtaining amounts of thermal expansion at a plurality of positions on the mask on the basis of an amount of light incident on the mask, so that the image-formation characteristic of the projection optical system is changed on the basis of the amounts of thermal expansion obtained.
The amounts of thermal expansion at a plurality of positions on the mask may be obtained on the basis of a pattern presence ratio on the mask.
The scanning exposure method may further include the step of obtaining an amount of change in image-formation condition of the pattern image due to thermal expansion of the mask in correspondence to each scanning position of the mask.
The scanning exposure method may further include the step of obtaining an amount of change in the image-formation characteristic of the projection optical system on the basis of an amount of light entering the projection optical system, so that the projection optical system is adjusted by taking into consideration the obtained amount of change in the image-formation characteristic of the projection optical system.
In addition, the present invention provides a scanning exposure method including the step of synchronously scanning a mask and a substrate relative to illuminating light, thereby transferring an image of a pattern on the mask onto the substrate by scanning exposure, and the step of changing, during the scanning exposure, a position of the mask in a direction of an optical axis of the illuminating light or a tilt of the mask relative to the optical axis according to a scanning position of the mask, thereby changing an image-formation condition of the pattern image.
In addition, the present invention provides a scanning exposure apparatus including a light source, and a scanning system for synchronously scanning a mask and a substrate relative to illuminating light from the light source. The apparatus further includes a projection optical system for projecting an image of a pattern on the mask onto the substrate, and a control system for changing, during scanning exposure, an image-formation characteristic of the projection optical system according to a scanning position of the mask.
The scanning exposure apparatus may further include an optical sensor for detecting an amount of light incident on the mask, and a system for calculating an amount of thermal expansion of the mask on the basis of the detected amount of light incident on the mask.
The control system may move a lens element of the projection optical system to change the image-formation characteristic of the projection optical system.
The control system may change a refractive index of a gas in a space provided in the projection optical system to change the image-formation characteristic of the projection optical system.